1. Field of the Invention
The present invention generally relates to laser scanning optical systems and, more particularly, to an achromatic-type laser scanning optical system for producing a high-resolution scanning image by means of a light source, such as a semiconductor laser or the like.
2. Description of the Related Art
A conventional type of laser scanning optical system is commonly arranged as shown in FIG. 1. A beam of laser light emanating from a semiconductor laser 1 is collimated by a collimator lens 2 and the collimated laser beam is focused on a reflecting surface 4a of a deflecting mirror 4 in linear form by a cylindrical lens 3. The cylindrical lens 3 has power in the sub-scan direction only (i.e., the direction perpendicular to the main-scan plane formed by a scanning beam during a scanning operation, or the direction parallel to the plane of the drawing of FIG. 1). Thereafter, the laser beam deflected by the deflecting mirror 4 is converged by a scanning lens assembly (f.multidot..theta. lens assembly) 5 so that a beam spot is projected on a photosensitive drum 6.
In general, the wavelength of oscillating light emanating from such a semiconductor laser tends to vary within a small wavelength bandwidth due to various factors such as temperature changes. For example, in a semiconductor laser of wavelength 780 nm, a wavelength variation of approximately .+-.20 nm occurs within a temperature range of -40.degree. C. to +60.degree. C. The position of the image plane formed by the laser scanning optical system also varies due to such wavelength variation.
A scanning optical system of the high-resolution type generally has a small f number (35 or less) so that it can form a micro beam spot. Accordingly, its focal depth, that is, a focal range within which a satisfactory beam spot can be produced, is extremely limited. As a result, if the image plane is varied in position as described above, the surface of the photosensitive drum may deviate from the satisfactory range of the focal depth and no beam spot of the required high resolution may be produced.
To cope with this problem, a number of approaches have already been proposed. A first known approach is to correct the chromatic aberration of a collimator lens so that the light beam transmitted through the collimator lens is consistently held as a parallel beam irrespective of the presence or absence of wavelength variation. A second known approach is to appropriately select the material of a lens barrel for accommodating a collimator lens so that physical variations in the lens barrel due to temperature changes cancel variations in the focal length of the collimator lens due to to wavelength variations. The second known approach is also intended to consistently keep the light beam transmitted through the collimator lens as a parallel beam irrespective of the presence or absence of wavelength variation. A third known approach is to install a detector for detecting the position of the image plane in a laser scanning optical system and a movable device for adjusting the position of the image plane. In the third approach, the movable device is driven in accordance with a signal supplied from the detector to adjust the position of the image plane so that the image plane is consistently held in an optimum position. A fourth known approach is to install a device for keeping a semiconductor laser warm at a constant temperature to prevent wavelength variations in the semiconductor laser.
However, any of the above-described known approaches involves a number of problems.
In each of the first and second approaches, the chromatic aberration of the scanning lens assembly (f.multidot..theta. lens assembly) is not taken into account. Accordingly, even if the collimator lens forms a parallel beam of light, the position of the image plane will vary due to the chromatic aberration of the scanning lens assembly itself. This is because the variation in the position of the image plane due to wavelength variation is not corrected by taking into account the entire optical system including the collimator lens and the scanning lens assembly. Particularly when a lens element made of optical glass with a high refractive index is incorporated in the scanning lens assembly, the dispersion of the optical glass used for the scanning lens assembly generally increases. As a result, the variation in the position of the image plane due to the chromatic aberration of the scanning lens assembly increases.
A certain scanning lens assembly is considered which has the numerical data shown in Table 1, a focal length f of 170.4 mm, and an optical arrangement such as that shown in FIG. 2. If the scanning lens assembly is used in combination with the collimator lens explained in connection with the first known approach, the resulting variation in the position of the image plane is as shown in FIG. 3. The scanning lens assembly can produce a satisfactory beam spot (a beam spot of 50 .mu.m in diameter with a semiconductor laser of wavelength 780 .mu.m) within a focal depth of .+-.1 mm, but the illustrated variation in the position of the image plane ranges over .+-.0.8 nm. For this reason, in the above known optical arrangement, it is necessary that each lens element of the scanning lens assembly be produced so that the positional accuracy of the entire scanning optical system with respect to the photosensitive drum 6 can be kept within .+-.0.2 mm. As a result, extremely strict working accuracy is required and the manufacturing cost thereof increases.
TABLE 1 ______________________________________ (Data on Scanning Lens Assembly 5) ______________________________________ R.sub.1 = -31.905 D.sub.1 = 4.70 N.sub.1 = 1.51072 R.sub.2 = -156.190 D.sub.2 = 2.095 N.sub.2 = 1 R.sub.3 = -107.660 D.sub.3 = 16.7 N.sub.3 = 1.76591 R.sub.4 = -52.701 D.sub.4 = 1.0 N.sub.4 = 1 R.sub.5.sup.( *.sup.1) = .infin. D.sub.5 = 16.1 N.sub.5 = 1.78569 R.sub.6.sup.( *.sup.1) = -131.56 f = 170.4 mm field angle = .+-.37.5.degree. f.sub.NO. = 4 wavelength = 780 nm ______________________________________ .sup.(*1) represents a toric lens, and the values of R.sub.5 and R.sub.6 with respect to the subscan direction are as follows: R.sub.5 = -157.46 R.sub.6 = -38.208
In Table 1, Ri represents the radius of curvature of the ith lens surface as viewed from the light-source side, Di represents the distance between the ith lens surface and the (i+1)th lens surface, and Ni represents the refractive index of a medium located at the rear of the ith lens surface. In FIG. 2, the right-hand side corresponds to the side on which the photosensitive drum 6 is located.
The third known approach has the advantage that it is possible to always accurately determine the position of an image plane. However, both the detector and the movable device are complicated in structure and the manufacturing cost of the entire apparatus therefore increases.
The fourth known approach is disadvantageous in that a means for detecting and controlling the temperature of the semiconductor laser is needed, in that a relatively long time is required until the temperature of the semiconductor laser reaches a predetermined temperature, and in that, if the apparatus is designed so that it can operate in various operational environments which involve, for example, a wide range of temperature changes, the apparatus will become expensive.